1. Field of the Present Invention
The present invention relates to a method of fabricating a T-gate, and more particularly, to a method of fabricating a micro T-gate of a high-speed semiconductor device using a photolithography method.
2. Discussion of Related Art
High frequency characteristics of high-frequency devices such as high electron mobility transistors (HEMT) are generally affected by gate length and resistance. Accordingly, a T-gate, which is short and has a low resistance and a large cross-sectional area, has to be used to fabricate a monolithic microwave integrated circuit (MMIC) which uses a high frequency of W band (75 to 110 GHz) or more. In general, an E-beam lithography method is used to fabricate a T-gate that is short and has a large cross-sectional area. Here, a double or triple photoresist layer is used. A conventional method of fabricating a T-gate will be described below with reference to the accompanying drawings.
FIGS. 1A to 1D are cross-sectional views illustrating a conventional method of fabricating a T-gate of a semiconductor device. First, a substrate 101 is prepared, and a first photoresist 102 is formed on the substrate 101. Then, a second photoresist 103 is formed on the first photoresist 102. The first and second photoresists 102 and 103 on the substrate 101 are formed of poly methyl methacrylate (PMMA), which can be classified according to transmissivity and loss of light. Here, the first photoresist 102 is formed of low sensitive PMMA, and the second photoresist 103 is formed of high sensitive PMMA, which is relatively more sensitive than the first photoresist 102. More specifically, the first photoresist 102 is applied and then a baking process is performed, and the second photoresist 103 is applied and then a baking process is also performed.
In the next process, referring to FIG. 1B, the second photoresist 103 is exposed (e-beam exposure) and then developed to form a head of the gate. This process is performed such that the gate head has a cross-section that is about 1 μm wide. The first photoresist 102 is patterned by an e-beam exposure process, and then a recess process is performed to form a relatively narrower gate leg than the gate head.
Referring to FIGS. 1C and 1D, a metal layer 104 is formed on the substrate 101 to form a gate electrode. The metal layer 104 is deposited on the entire surface of the exposed substrate 101 using an e-beam evaporator, and then a lift-off process is performed to remove the first and second photoresists 102 and 103. Thus, the T-gate electrode 105 is completed.
However, in the case of forming the T-gate electrode according to the conventional art, the gate may not be made shorter than the case of patterning the gate head only using the low sensitive PMMA layer. And, in exposure and development of the high sensitive PMMA, since the low sensitive PMMA layer that is under the high sensitive one is exposed, it is not easy to exactly adjust the gate length. Moreover, since the exposure and development of the photoresists use an electron beam, processing time and production cost rise.
Another method of fabricating a T-gate, which solves theses problems, is illustrated in FIGS. 2A to 2F. Referring to FIGS. 2A to 2F, a low sensitive PMMA is applied to form a first photoresist layer 202 on a substrate 201, and then a baking process is started. Then, the first photoresist layer 202 is exposed (by e-beam) and developed to form a leg of a gate. A second image reversal photoresist layer 203 is applied and then a baking process is performed. Subsequently, exposure is performed by photolithography to form a head of the gate, which has a large cross-sectional area. Then, the second image reversal photoresist layer 203 is patterned. Referring to FIG. 2E, a metal layer 204 is deposited to form a gate electrode on the substrate 201. The metal layer 204 is deposited on the entire surface of the exposed substrate 201 using an e-beam evaporator, and the first and second photoresist layers 202 and 203 are lifted off, and thus the T-gate electrode 205 is formed.
Since the method described above uses photolithography, processing time may be reduced. However, in the method, the gate leg which has a great impact on characteristics of the high frequency device is patterned first, and then the gate head is patterned by applying the image reversal photoresist layer, so a photoresist residue may remain under the patterned gate leg, which is not easy to be removed. Moreover, for this reason, the gate length may not be uniform.